Time-resolved direct carbonation of fresh concrete slurry waste for carbon capture and storage based on hydrochemical parameters, mineralogy, and micromorphology

Concrete slurry waste (CSW), a by-product of concrete processing, is a substantial yet underexploited CO 2 sink. During accelerated carbonation of a model CSW we previously observed a pronounced and reproducible minimum in the electric conductivity (EC min ) shortly after initiating carbonation. With sustained supply of CO 2 and while the pH fell below 10, the EC rebounded. Here, we investigated if this was caused by redissolution of CaCO 3 which would limit the carbonation efficiency. To test this, slurry carbonation experiments were conducted with model slurry and real CSW in a stirred tank carbonation reactor with gas recirculation. Online monitoring of pH, EC, and reactor pressure was complemented by stage-resolved sampling, including the EC min , for analysis of the solid and aqueous phases along the treatment. Other than hypothesized, in both materials CaCO 3 precipitation continued beyond EC min , yet at a lower rate as confirmed by the time-course of reactor pressure and thermogravimetric analysis. The rebounding EC beyond EC min was largely driven by increasing sulfate concentrations. These may date back to interlayer sulfate released upon carbonation of C–S–H and/or to dissolution of ettringite-borne gypsum. Thus, carbonation of cement hydration products likely controlled the CO 2 uptake below pH 10. Our results indicate that online monitoring of the robust process indicators pH and EC provides valuable insight into the carbonation progress and may serve to delineate the appropriate termination point according to specific treatment objectives.